guix build: Add '--log-file'.

* guix/scripts/build.scm (show-help): Add '--log-file'.
  (%options): Likewise.
  (guix-build): Set %FILE-PORT-NAME-CANONICALIZATION.  Honor '--log-file'.
* tests/guix-build.sh: Add '--log-file' tests.
* doc/guix.texi (Invoking guix build): Document '--log-file'.

doc/guix.texi

@@ -1546,6 +1546,22 @@ Use the given verbosity level.  @var{level} must be an integer between 0
 and 5; higher means more verbose output.  Setting a level of 4 or more
 may be helpful when debugging setup issues with the build daemon.
 
+@item --log-file
+Return the build log file names for the given
+@var{package-or-derivation}s, or raise an error if build logs are
+missing.
+
+This works regardless of how packages or derivations are specified.  For
+instance, the following invocations are equivalent:
+
+@example
+guix build --log-file `guix build -d guile`
+guix build --log-file `guix build guile`
+guix build --log-file guile
+guix build --log-file -e '(@@ (gnu packages guile) guile-2.0)'
+@end example
+
+
 @end table
 
 Behind the scenes, @command{guix build} is essentially an interface to

guix/scripts/build.scm

@@ -95,6 +95,8 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
                          as a garbage collector root"))
   (display (_ "
       --verbosity=LEVEL  use the given verbosity LEVEL"))
+  (display (_ "
+      --log-file         return the log file names for the given derivations"))
   (newline)
   (display (_ "
   -h, --help             display this help and exit"))
@@ -161,7 +163,10 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
                 (lambda (opt name arg result)
                   (let ((level (string->number arg)))
                     (alist-cons 'verbosity level
-                                (alist-delete 'verbosity result)))))))
+                                (alist-delete 'verbosity result)))))
+        (option '("log-file") #f #f
+                (lambda (opt name arg result)
+                  (alist-cons 'log-file? #t result)))))
 
 
 ;;;
@@ -235,6 +240,9 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
              (leave (_ "~A: unknown package~%") name))))))
 
   (with-error-handling
+    ;; Ask for absolute file names so that .drv file names passed from the
+    ;; user to 'read-derivation' are absolute when it returns.
+    (with-fluids ((%file-port-name-canonicalization 'absolute))
       (let ((opts (parse-options)))
         (define package->derivation
           (match (assoc-ref opts 'target)
@@ -251,6 +259,9 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
                                       str package->derivation sys src?))
                                     (('argument . (? derivation-path? drv))
                                      (call-with-input-file drv read-derivation))
+                                    (('argument . (? store-path?))
+                                     ;; Nothing to do; maybe for --log-file.
+                                     #f)
                                     (('argument . (? string? x))
                                      (let ((p (find-package x)))
                                        (if src?
@@ -265,9 +276,10 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
                                      (_ #f))
                                     opts)))
 
+            (unless (assoc-ref opts 'log-file?)
               (show-what-to-build (%store) drv
                                   #:use-substitutes? (assoc-ref opts 'substitutes?)
-                              #:dry-run? (assoc-ref opts 'dry-run?))
+                                  #:dry-run? (assoc-ref opts 'dry-run?)))
 
             ;; TODO: Add more options.
             (set-build-options (%store)
@@ -278,13 +290,27 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
                                #:max-silent-time (assoc-ref opts 'max-silent-time)
                                #:verbosity (assoc-ref opts 'verbosity))
 
-          (if (assoc-ref opts 'derivations-only?)
+            (cond ((assoc-ref opts 'log-file?)
-              (begin
+                   (for-each (lambda (file)
+                               (let ((log (log-file (%store) file)))
+                                 (if log
+                                     (format #t "~a~%" log)
+                                     (leave (_ "no build log for '~a'~%")
+                                            file))))
+                             (delete-duplicates
+                              (append (map derivation-file-name drv)
+                                      (filter-map (match-lambda
+                                                   (('argument
+                                                     . (? store-path? file))
+                                                    file)
+                                                   (_ #f))
+                                                  opts)))))
+                  ((assoc-ref opts 'derivations-only?)
                    (format #t "~{~a~%~}" (map derivation-file-name drv))
                    (for-each (cut register-root <> <>)
                              (map (compose list derivation-file-name) drv)
                              roots))
-              (or (assoc-ref opts 'dry-run?)
+                  ((not (assoc-ref opts 'dry-run?))
                    (and (build-derivations (%store) drv)
                         (for-each (lambda (d)
                                     (format #t "~{~a~%~}"
@@ -299,4 +325,4 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
                                          (map cdr
                                               (derivation->output-paths drv)))
                                        drv)
-                                 roots)))))))))
+                                  roots))))))))))

tests/guix-build.sh

@@ -36,6 +36,17 @@ guix build -e '(@@ (gnu packages base) %bootstrap-guile)' |	\
 guix build hello -d |				\
     grep -e '-hello-[0-9\.]\+\.drv$'
 
+# Should all return valid log files.
+drv="`guix build -d -e '(@@ (gnu packages base) %bootstrap-guile)'`"
+out="`guix build -e '(@@ (gnu packages base) %bootstrap-guile)'`"
+log="`guix build --log-file $drv`"
+echo "$log" | grep log/.*guile.*drv
+test -f "$log"
+test "`guix build -e '(@@ (gnu packages base) %bootstrap-guile)' --log-file`" \
+    = "$log"
+test "`guix build --log-file guile-bootstrap`" = "$log"
+test "`guix build --log-file $out`" = "$log"
+
 # Should fail because the name/version combination could not be found.
 if guix build hello-0.0.1 -n; then false; else true; fi